1. Field of the Invention
The present invention relates to welded cans making use of a surface-treated steel sheet plated with tin as a raw material and having a protective coat on at least the inner surface of a can body.
2. Description of the Related Art
In recent years, welded cans have come to be used as food or beverage cans. Tinned steel sheets (tinplate sheets) are generally used for such welded cans because they can be welded at a low temperature. Tin deposit is also known to have a function of protecting a base steel sheet against corrosion by contents. As such tinned steel sheets, those having a tin deposition rate of at least 2.8 g/m.sup.2 and used for soldered cans have generally been used to date. However, there has recently been a tendency to use steel sheets low in tin deposition rate as raw materials for welded cans because the price of tin has been tending upward for reasons of resources.
However, a welded can making use of a steel sheet whose tin deposition rate is lowered is accompanied by the following problem. When such a steel sheet is subjected to a heat treatment such as coating or printing in a can processing step, tin plated on a base steel sheet fuses to form a tin-iron alloy, which has a high melting point and is hard to weld, together with the base steel sheet, so that the amount of pure tin required for welding is decreased, and the strength of the can at a welded joint hence becomes insufficient. The reduction of the tin deposition rate is also accompanied by a problem that the corrosion resistance against the contents of the welded can is lowered.
Therefore, it has been proposed to achieve excellent weld quality while lowering the tin deposition rate and at the same time, to form a protective coat on a portion of the tinned steel sheet, which corresponds to at least an inner surface of a can body, with the exception of both ends, thereby aiming at ensuring the corrosion resistance of the welded can.
For example, as described in Japanese Patent Publication No. 13896/1990, the present inventors proposed a welded can obtained by using a tinned steel sheet having a tin layer obtained by evenly plating tin on the surface of a base steel sheet at a deposition rate of 0.5-1.7 g/m.sup.2, and forming a coating layer composed of a thermosetting epoxy-phenolic resin coating on a portion of the surface-treated steel sheet, which corresponds to at least the inner surface of a can body, with the exception of both ends. On the base steel sheet of the welded can, at least two layers of a tin-iron alloy layer having a deposition rate of 0.35-1.60 g/m.sup.2 in terms of tin content and a tin layer having a tin deposition rate of 0.10-1.35 g/m.sup.2 are formed in that order at a stage after baking for coating but before welding.
At the stage before welding, the base steel sheet securely holds the tin layer on its surface at a tin deposition rate of 0.10-1.35 g/m.sup.2. Therefore, it can obtain sufficient weld strength. The reduction of corrosion resistance against the contents of the welded can due to the lowering of the tin deposition rate is also made up by the coating layer composed of the thermosetting epoxy-phenolic resin coating.
According to a further study of the present inventors, it is however apprehended that since the coating layer is a coating film formed by applying the thermosetting epoxy-phenolic resin coating and baking it, iron may dissolve out in the contents from pin-holes if any pin-holes generate upon the application of the coating, and low-molecular weight components in the coating film may affect the flavor of the contents.
As described in Japanese Patent Application Laid-Open No. 232148/1986, the present inventors also proposed a welded can obtained by using a tinned steel sheet having a tin-iron alloy layer, a tin layer and a chromium.chromate film in that order on the surface of a base steel sheet and forming a coating layer composed of a thermosetting epoxy-phenolic resin coating on a portion of the surface-treated steel sheet, which corresponds to at least the inner surface of a can body, with the exception of both ends. On the base steel sheet of the welded can, a tin layer having a tin deposition rate of at least 0.05 g/m.sup.2 and distributed in a range of 10-60% of the surface area of the base steel sheet is formed at a stage after baking for coating but before welding. This tin layer is formed in a mottled or islandlike state.
At the stage before welding, the base steel sheet described in the above official gazette securely holds on its surface a tin layer having a tin deposition rate of at least 0.05 g/m.sup.2 and formed in a mottled or islandlike state. Therefore, it can obtain sufficient weld strength. The reduction of corrosion resistance against the contents of the welded can due to the lowering of the tin deposition rate is also made up by the coating layer composed of the thermosetting epoxy-phenolic resin coating.
It is however apprehended that since the coating layer is composed of a thermosetting epoxy-phenolic resin coating like that described in Japanese Patent Publication No. 13896/1990, iron may dissolve out in the contents, and the flavor of the contents may be adversely affected.